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Just want to know whether ATC radar always reads aircraft height with respect to standard altitude setting or will read the change in height if we select QNH instead of standard 1013 hPa (QNE)?

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  • $\begingroup$ Wait a second... QNH altitude is based on a "ground" (sea level) pressure of 1013 hPa, no? QFE altitude varies according to air pressure at local ground (with "local ground" always being an altitude of zero, whatever that means in terms of air pressure). $\endgroup$ – a CVn Dec 17 '17 at 19:16
  • $\begingroup$ @MichaelKjörling: QNH altitude is based on a "ground" (sea level) pressure of 1013 hPa, no? No, QNH is assumed pressure at sea level (computed from QFE). QNE is 1013.25 hPa as indicated in the question. $\endgroup$ – mins Dec 17 '17 at 21:32
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The altitude reported by the mode C of the transponder is based on the standard altimeter setting (29.92 in the United States)

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  • $\begingroup$ sir do you mean that if pilot changes its altimeter setting even then the height send by transponder to radar scope would be that of standard 29.92? $\endgroup$ – abbasi Dec 17 '17 at 16:27
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    $\begingroup$ @abbasi: Yes. This makes the reported altitude unambiguous even when the QNH is lower than standard, and eliminates the possibility of separation errors caused by pilots setting the altimeter wrong. The ATC computers can convert the result so it is displayed to controllers with QNH corrections for aircraft below whatever the local transition altitude is. $\endgroup$ – Henning Makholm Dec 17 '17 at 16:35
  • $\begingroup$ @abbasi: Some altitude encoders (blind encoders) have no connection with the cockpit, hence couldn't be set by the crew anyway. Requirement for encoders are described in TSO-C88. $\endgroup$ – mins Dec 17 '17 at 21:39

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